1. Field of the Invention
This invention relates to an improved process of preparing di-t-octyldiazene by oxidizing N,N'di-t-octylsulfamide with sodium hypochlorite and caustic and optionally in the presence of a phase transfer catalyst and/or t-butyl alcohol at a temperature of 65.degree.-90.degree. C. in a minimal amount of solvent.
2. Prior Art
Among the many t-azoalkanes (I) known, ##STR1## di-t-octyldiazene (II) is a particularly attractive compound to make commercially. ##STR2## Di-t-octyldiazene is a known compound whose rate of thermolysis was studied by Timberlake and co-workers (B. K. Bandlish, A. W. Garner, M. L. Hodges and J. W. Timberlake, J. Am. Chem. Soc. 97, p.5856-5862, 1975). From their thermolysis data the 10 hr. half-life temperature (t1/2) was calculated to be about 107.5.degree. C. Thus, its decomposition temperature falls into a very useful temperature range for initiating vinyl polymerizations. The compound is a liquid at room temperature and is relatively non-volatile. Since it does not contain any cyano groups as do the commercial symmetrical and unsymmetrical azonitriles, it should not generate any toxic residues upon decomposition. Despite the attractiveness of this compound, no evidence was found that di-t-octyldiazene (II) is being produced or used commercially despite the commercial availability of t-octylamine. This is because prior to the present invention there wasn't any commercially feasible route to prepare di-t-octyldiazene.
In 1965 R. Ohme and E. Schmitz developed a general synthetic method for the preparation of azoalkanes (R. Ohme and E. Schmitz, Angew Chem Int. Ed. Engl. 4, p.433, 1965). They found that the dialkylamides of sulfuric acid in a solution of N alkali react with 2 moles of NaOCl at 20.degree.-60.degree. C., to form aliphatic azo compounds. They prepared the low molecular weight azopropane, azobutane, and azocyclohexane in this manner.
In 1967 R. Ohme and H. Preuschof studied the mechanism of this oxidation as well as the oxidation of N,N'-disubstituted sulfamides and monosubstituted sulfamides (R. Ohme and H. Preuschkof, Liebigs Ann. Chem. 713, p.74-86, 1968). During the course of their investigation they prepared 2,2'-azoisobutane in 84% yield by oxidizing N,N'-di-tert-butylsulfamide in 2N NaOH with 2 equivalents of NaOCl at 60.degree. C. J. C. Stowell prepared 2,2'-azoisobutane in 84% yield by running a similar type reaction for 3 hours in pentane (J. C. Stowell, J. Org. Chem. 32, p.2360, 1967).
In 1972 J. W. Timberlake and co-workers attempted to prepare di-t-octyldiazene and di-t-heptyldiazene by this route but were unsuccessful (J. W. Timberlake, M. L. Hodges and K. Betterton, Synthesis 1972, p.632-34). Treatment of either N,N'-bis[2,4,4-trimethyl-2-pentyl]sulfamide (i.e. N,N'-di-t-octylsulfamide) or N,N'-bis[2,3,3-trimethyl-2-butyl]sulfamide (i.e. N,N'-di-t-heptylsulfamide) under the conditions specified in Ohme's articles gave no azo compound. Timberlake recognized that the oxidation of sulfamides to azo compounds was not adaptable to all azos. The conditions were too vigorous for isolating unstable azo compounds and solubility problems in several cases led to quantitative return of starting sulfamides. Therefore Timberlake developed a more complex method of converting the N,N'-dialkylsulfamides into azoalkanes. He used a completely homogeneous mixture with potassium t-butoxide as the base, t-butyl hypochlorite as the chlorinating agent, and t-butanol as the solvent. He also developed a heterogeneous mixture with sodium hydride as the base and t-butyl hypochlorite as the chlorinating agent in an ether/pentane solvent. Timberlake prepared di-t-octyldiazene in 78% yield by treating the sulfamide with a slurry of 2 eqs. of sodium hydride in pentane for 2 hours at room temperature, the reaction cooled to 0.degree. C. and 2 eqs. of t-butyl hypochlorite added dropwise and the mixture stirred overnight. The excess sodium hydride was then destroyed by the careful addition of water and the pentane solution of the azo chromatographed over alumina. The azo was then distilled to obtain a 78% yield. Although the process was very useful for preparing laboratory scale samples, it was hardly practical for development on a commercial scale.
In 1974 C. Ruchardt and co-workers proclaimed that they had a new simple, high yield procedure for the large scale synthesis of tert-azoalkanes (I) from readily available chloroazoalkanes and trialkyl- or triphenylaluminum (W. Duismann, H. Beckhaus and C. Ruchardt, Liebigs Ann. Chem. 1974, p.1348-1356). Ruchardt stated that the t-azoalkanes are excellent generators of free radicals but representatives of this class of compounds have still not acquired any significance as initiators in industry solely due to the difficulty in preparing them. The known syntheses are tedious and frequently produce low yields. None of the known preparative procedures are suited to general large-scale synthesis of t-azoalkanes. Ruchardt felt his process would overcome these shortcomings. Ruchardt and co-workers produced 28 different symmetrical t-azoalkanes by this process. They prepared di-t-octyldiazene in 82% yield.
In 1976 M. Prochazka prepared di-t-octyldiazene by the oxidation of t-octylamine with IF.sub.5 (M. Prochazka, Collect. Czech. Chem. Commun. 1976, 41(5), p.1557-1564 (Eng); C.A. 86, p.338c, 1977). The compound was prepared on a small scale for comparison of its rate of decomposition with other t-azoalkanes Prochazka prepared.
In 1978 a Japanese patent described a process for oxidizing N,N'-di-t-octylsulfamide with bleach and caustic in the presence of a phase transfer catalyst (Japan Kokai 77, 128, 305; C.A. 88, p.120602m, 1978). The reaction was run at 40.degree. C. and required 10 hours to complete. This was the first indication that di-t-ocyldiazene could be prepared by the aqueous bleach route. The process required a phase transfer catalyst and a long reaction period.
In early 1982 a European patent (European Pat. No. 0,006,972) was published describing a photopolymerization process. In the patent, there is a description of the preparation of 2,2'-azobis(2,4,4-trimethylpentane) which is also referred to as di-t-octyldiazene. The di-t-octyldiazene was prepared by oxidizing N,N'-di-t-octylsulfamide with bleach and caustic solution for 20 hours at 35.degree. C. The yield was only 50% after purification by vacuum distillation. A complete description of the experiment was not provided. However, in other examples in the patent, 2,2'-azobis-2-methylbutane and 1,1'-di-methyl-azocyclopentane were prepared by oxidizing the corresponding sulfamides with sodium hypochlorite solution in the presence of 10 parts pentane to 1 part t-butyl alcohol. The reactions were stirred for 24 hours at 35.degree.-40.degree. C. Specific % yields were not reported.
In 1971 A. Ohno and co-workers reported the preparation of azobis-(2-propyl)-2-propane by sodium hyprochlorite oxidation of the corresponding sulfamide (A. Ohno, N. Kito and Y. Ohnishi, Bull. Chem. Soc. Japan, 1971, 44, p.470-474). The azo was obtained in 38% yield after stirring 7.0 grams of the sulfamide in 50 ml of hexane with 150 ml of 10% NaOCl for 35 hours at room temperature. The crude product was purified by distillation.
Although these reactions used cheap raw materials, i.e. sodium hydroxide and sodium hypochlorite instead of sodium hydride and t-butyl hypochlorite, the reaction time required to complete the reaction was much too long to be commercially attractive. Therefore, despite the fact that di-t-octyldiazene has been made by five various routes, there is no commercially attractive route to di-t-octyldiazene. In addition, there is no indication that the oxidation of the sulfamide can be carried out in high yield and short reaction time using the sodium hypochlorite-sodium hydroxide system. In fact the prior art indicates it cannot be done.